The present invention relates to electro-optic displays and methods for driving such displays. Certain aspects of the present invention are directed especially to electrochromic displays, and more specifically to (i) electrochromic displays with solid charge transport layers; (ii) apparatus and methods for improving the contrast and reducing the cost of electrochromic displays; and (iii) apparatus and methods for sealing electrochromic displays from the outside environment and preventing ingress of contaminants into such a display.
Electro-optic displays are used in a wide variety of devices for displaying text, still image graphics, and moving pictures. (The term “electro-optic display” is used herein in its conventional meaning in the art to refer to a display using an electro-optic material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.) Devices that currently use electro-optic displays include digital wristwatches., calculators, personal digital assistants (PDA's), flat screen computer displays, laptop personal computers, and cellular phones. As the electro-optic display has evolved into an important and versatile interface to modern electronic appliances, the microelectronics industry and the display technology industry have formed a powerful technology partnership in developing new applications. The display industry has continually introduced new technologies for improved electro-optic display performance and the microelectronics industry has followed with the hardware and software to support these new displays.
The first electro-optic displays, including Nixie-tubes, were expensive and fragile, were severely limited in the data they could present, and required a significant amount of power, space, and support electronics. This hampered both their usefulness and acceptance. With the advent of the solid-state light emitting diode (LED), the cost, size, and circuit complexity for electro-optics were reduced and the technology was widely accepted, especially in numeric display applications. Control circuitry was easily implemented with existing digital circuit techniques but still required high power drive stages. Since LED's still drew a significant amount of power and could not easily scale to meet the increasing market demand for higher resolution displays, display development moved to new approaches.
In searching for a better method, investigation focused on optical properties of liquid crystals. U.S. Pat. No. 3,932,024, assigned to Dai Nippon Toryo Kabushiki Kaisha (Osaka, Japan), describes a liquid crystal display device comprising a pair of opposed electrode-mounted plates, each of which is provided with a polarizer, the planes of the two polarizers being perpendicular to each other, and a nematic liquid crystal layer. The electrode terminals are mounted on one plate by transferring the connection of one electrode to the opposite plate without directly contacting the liquid crystal material by interposing an electrically conductive material between the corresponding electrode terminal and the one electrode.
Common-plane-based LCD's are good for simple displays that need to show the same information over and over again. Watches and microwave timers fall into this category. Although a hexagonal bar shape is the most common form of electrode arrangement in such devices, almost any shape is possible. Examples of some of the electrode shapes defined in applications such as inexpensive handheld games include playing cards, aliens, fish, and slot machines.
Passive-matrix LCD's use a simple grid to supply the charge to a particular pixel on the display. The grids are formed on top and bottom glass layers called substrates. One substrate forms the “columns” and the other substrate forms the “rows”. The wiring of the column or rows is made from a transparent conductive material, usually indium-tin oxide (ITO). The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, the integrated circuit sends a charge to the correct column of one substrate and electrically grounds the associated row where the intersection of the row and the column will determine the “pixel” or cell element to be activated. The row and column intersect at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that pixel.
The passive-matrix system is simple and cost effective, but it has significant drawbacks, notably slow response time and imprecise voltage control. Response time refers to the LCD's ability to create or recreate (refresh) the image displayed. Slow response time is especially noticeable in pointer- or mouse-driven graphical user interfaces. In addition, imprecise voltage control hinders the passive matrix's ability to influence only one pixel at a time. When voltage is applied to untwist one pixel, the pixels around it also partially untwist, which makes images appear fuzzy. Therefore, each pixel lacks contrast with its neighboring pixel.
The active-matrix LCD was developed to ameliorate many of the limitations of the passive-matrix display. In this type of LCD display, the addressing takes place completely behind the liquid crystal film. The front surface of the display is coated with a continuous electrode while the rear surface electrode is patterned into individual pixels. A thin film transistor (TFT) acts as a switch for each pixel. The TFT is addressed by a set of narrow multiplexed electrodes (gate lines and source lines) running along the gaps between pixels. A pixel is addressed by applying current to a gate line that switches the TFT on and allows a charge from the source line to flow on to the rear electrode. This sets up a voltage across the pixel and turns it on. An image is created similar to the passive display as the addressing circuitry scans across the matrix. By controlling the amount of voltage supplied to a pixel crystal, the amount of crystal twist can be controlled. By doing this in exact, minute increments, active-matrix LCD's can display usable gray scale images. The active-matrix display technology offers improved response time, viewing angle, contrast, and intensity control as compared with passive-matrix LCD's. Hence active matrix displays are the technology of choice for high-resolution electro-optic computer applications.
In all LCD displays, the liquid crystal material is activated by a discrete applied voltage (e.g., 5 volts) and the liquid crystal changes its optical properties in response to that voltage. When the voltage is removed from the cell, the liquid crystal returns to its original state. The hardware that drives the LCD can consist of simple combinatorial digital logic. Slightly more complex circuits can be used that take into account knowledge of specific display performance variables, such as cell transition times or ambient temperature response. When different levels of twist are required, i.e., gray scale, control of the applied voltage level is required, necessitating more complex drive circuitry and knowledge of the optical properties versus voltage curve.
To provide full color in an LCD, each individual pixel is divided into three sub-pixels, normally red, green, and blue (RGB). Applying color filters that only allow certain wavelengths to pass through them while absorbing the rest creates these sub-pixels. With a combination of red, blue, and green sub-pixels of various intensities, a pixel can be made to appear any number of different colors. The number of colors that can be made by mixing red, green, and blue sub-pixels depends on the number of distinct gray scale levels (intensities) that can be achieved by the display.
The liquid crystal-based electro-optic display industry has been subject to the same cost/performance market pressure as the microelectronics industry. As a result, LCD's have been following a trend towards increased density, improved color depth, faster response time, and lower cost. The hardware and software used to control these voltage-driven devices are well known, well characterized and relatively simple, thus allowing for ease of hardware/software/display integration, and this has contributed to widespread adoption of the technology.
However, LCD's do have some inherent drawbacks. Transmissive LCD's require backlighting, which draws significant power. Also, the contrast, though much improved over early implementations, is inherently limited to the background and foreground colors and color differentiation. Reflective LCD's (which essentially place a reflector on the opposed side of the display from the observer) have insufficient contrast for many applications. Typically, LCD's require special packaging to keep the liquid crystal in a predetermined region in each cell and in the overall display. Furthermore, since the extent of rotation of the plane of polarization of light by the liquid crystal depends upon the thickness of liquid crystal layer, this thickness must be accurately maintained, which renders it very difficult to prepare LCD's on flexible substrates.
One type of non-liquid crystal electro-optic display, namely organic light emitting diode (OLED) displays, has found favor where low power, high contrast, and fast response times are required. The OLED technology has shown great potential but limited commercial viability, due to some significant performance issues.
OLED's are usually arranged in an active-matrix arrangement, similar to that used in LCD's. However, the electrical requirements of the OLED pixel circuit are significantly different. This circuit must provide a constant current to the OLED device, but the magnitude of the current must be controllable over a range of more than two orders of magnitude in order to allow for high-contrast images. Typically, in active-matrix OLED displays, there are two metal oxide semiconductor field effect transistor (MOSFET) drivers at each pixel. A voltage is applied for a set period of time to the first transistor, causing it to turn on and conduct, storing a charge on a capacitor. This capacitor then connects to the gate of a second transistor, and causes the latter to conduct charge to the OLED pixel, a process that continues until another signal is applied to discharge the capacitor. Thus, the OLED emits continuously at an intensity defined by the rate of charge flow (current).
The current source is one of the two critical components of the OLED pixel cell, and its design is set by the actual pixel current requirement. This requirement in turn is derived from the target luminance, the OLED efficiency, the color filter transmission (when used), the relative and absolute areas of the color sub-pixels, and the duty cycle of the pixel.
The second most important component of an active-matrix OLED pixel cell is the storage element. Even though an OLED is current driven, the most practical way to store energy is the capacitor. Fortunately, a MOSFET is a fairly good voltage-to-current converter, when driven properly, so that, in essence, each pixel is driven by a current driver that is controlled externally by an applied voltage.
In many ways, the OLED has the same benefits of external drive circuit simplicity as the active-matrix LCD. However, many technological challenges lie ahead before OLED's can become a commercially viable display technology; these challenges include short operational life and susceptibility to moisture, which degrades the displays, and requires the use of special hermetically sealed packages. Indeed, some modern OLED's are sealed in “can packages” with desiccants inside the package to absorb moisture. This type of packaging increases the cost of the overall displays and severely limits the use of these displays, since the hermetically sealed packing is difficult to scale down in thickness and therefore is not useable for very thin display devices (credit card type displays, flexible displays, etc.). It is also difficult to scale OLED displays to large sizes because of high defect densities and the technical difficulties associated with making OLED's in large area form factors.
Another type of electro-optic display is a particle-based electrophoretic display, described for example in U.S. Pat. Nos. 3,668,106; 3,756,693; and 3,767,792. Particle-based electrophoretic displays make use of one or more types of electrically-charged particles dispersed in a suspending fluid. Known electrophoretic media can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspended in a colored suspending medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode, so that the particles are hidden by the colored suspending medium).
A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles are black and the other type white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer.
If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility, will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid be sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in such a display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough.
Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate the two extreme optical states already described. (The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel of a display, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned, the transition between the two extreme states may not be a color change at all.)
Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, optical state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in United States Published Patent Application 2002/0180687 that some particle-based electro-optic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” will be used herein to cover both bistable and multi-stable displays.) Nevertheless, problems with the long-term image quality of electrophoretic displays such as those described in the three aforementioned patents have prevented their widespread usage. For example, the dispersed particles used in electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; and 6,531,997; and U.S. patent applications Publication Nos. 2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0154382, 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011867; and 2003/0025855; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/20922; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; and WO 01/17029.
Examples of suitable molten salts include trifluoromethanesulfonates and bis-trifluoromethylsulfonylamidures. 1-Propyl-dimethyl imidazolium trifluoro and lithium perchlorate are particularly prefered.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, WO 01/02899, at page 10, lines 6-19. See also the aforementioned 2002/0131147. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
It should be noted that, although electrophoretic displays are often opaque (since the particles substantially block transmission of visible light through the display) and operate in a reflective mode, electrophoretic displays can be made to operate in a so-called “shutter mode” in which the particles are arranged to move laterally within the display so that the display has one display state which is substantially opaque and one which is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Applications Publication No. WO 02/01281, and published U.S. application Ser. No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Unlike the previous discussed display technologies, particle-based electrophoretic displays act, to a first approximation, as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.
Writing/clearing pixels in particle-based electrophoretic displays, or changing gray scale in such pixels involves switching voltages on and off or applying opposite voltages to move the appropriate particles into the desired position. Knowledge of (1) the initial state of the pixel, (2) the time required to move the particles, i.e., the transition time, (3) the time at voltage curve vs. optical properties, and (4) the relaxation time of the pixel is important to provide a high quality image on a particle-based electrophoretic display.
Color electrophoretic displays can be implemented using red/green/blue particles. To accomplish this, either each particle type needs to react to a different voltage level or each colored particle within a pixel would require a separate sub-pixel.
Another type of electro-optic display similar to a particle-based electrophoretic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface.
Finally, another type of electro-optic display is an electrochromic display; this type of display uses a material which changes at least one optical characteristic as electrons are added thereto or removed therefrom. (Electrochromism is defined as a reversible color change of a material caused by the application of an electrical current.) Several types of electrochromic displays are known. According to U.S. Pat. No. 6,301,038 (derived from International Application PCT/IE98/00008, Publication No. WO 98/35267), one type of electrochromic display is based on ion insertion reactions at metal oxide electrodes. To ensure the desired change in transmittance the required number of ions must be intercalated in the bulk electrode to compensate the accumulated charge. However, use of optically flat metal oxide layers requires bulk intercalation of ions as the surface area in contact with electrolyte is not significantly larger than the geometric area. As a consequence the switching times of such a device are typically of the order of tens of seconds.
Also according to the same U.S. Pat. No. 6,301,038, a second type of electrochromic display is based on a transparent conducting substrate coated with a polymer to which is bound a redox chromophore. On applying a sufficiently negative potential, the electrochromic polymer is typically oxidized from its neutral state to a deeply colored form, the color of which depends upon the nature of the polymer. Among the electrochromic polymers which may be used in this type of display are polythiophenes, polypyrroles, and polyanilines. To ensure the desired change in transmittance a sufficiently thick polymer layer is required, the latter implying the absence of an intimate contact between the transparent conducting substrate and a significant fraction of the redox chromophores in the polymer film. As a consequence the switching times of such a device are, as above, typically of the order of tens of seconds.
The aforementioned U.S. Pat. No. 6,301,038 describes an electrochromic display in which the active layer (i.e., the layer whose optical characteristics are varied by addition or removal of electrons) is a nanoporous-nanocrystalline film comprising a semi-metallic oxide having a redox chromophore adsorbed thereto. In the related WO 01/27690 (see also Wood, D., “An Electrochromic Renaissance?”, Information Display, 18(3), 24 (March 2002, hereinafter referred to as the “Wood article”—the entire disclosure of this article is herein incorporated by reference) broadens the concept to a nano-porous, nano-crystalline film comprising a conducting metal oxide having an electroactive compound which is either a p-type or n-type redox promoter or p-type or n-type redox chromophore adsorbed thereto. Furthermore, the present invention further broadens the concept by removing the limitation to adsorption of the electroactive compound, and provides that this compound may be chemically bonded to the metal oxide. The present invention extends to displays using this type of chemically-bonded electroactive compound, as well as to displays using solid electrolytes, as described in more detail below. Although most aspects of the present invention will primarily be described with specific reference to embodiments of the type described in the aforementioned U.S. Pat. No. 6,301,038, the necessary modifications to embodiments of the types described in the aforementioned WO 01/27690 and Wood article and the types using solid electrolytes will readily be apparent to those skilled in the technology of electrochromic displays.
For an electronic display, the electrochromic effect is only useful if the color change is truly reversible. Typically, a current flow in one direction causes a color to form, while reversing the current flow causes the color to disappear (bleach). Materials showing this effect are known as electrochromic and may be organic (carbon-based) or inorganic in character.
FIG. 1 of the accompanying drawings shows a schematic cross-section through a display (generally designated 100) described in the aforementioned U.S. Pat. No. 6,301,038. The display 100 comprises a first glass substrate 102, a first fluorine-doped tin-oxide coated conductive layer 104, a nano-structured film 106 of titania (TiO2) coated on the first conductive layer 104, a redox chromophore 108 adsorbed on the titania in the film 106, an electrolyte or electron donor solution 110, a second conductive layer 112 of fluorine-doped tin oxide, and a second glass substrate 114. The redox chromophore can be any of a variety of N,N-disubstituted derivatives of 4,4′-bipyridyl, the preferred one being N,N′-bis(2-phosphonoethyl)-4,4′-bipyridinium dichloride (referred to as bis(2-phosphonoethyl)-4,4′-bipyridinium dichloride in the aforementioned U.S. Pat. No. 6,301,038).
The aforementioned Wood article describes what is apparently a later variation of the same process, in which the substrates 102 and 114 are formed of glass coated with indium tin oxide (ITO), as used in LCD's. The first substrate 102 (which forms the front electrode in the final display) is coated with the same anatase titania/redox chromophore layer, the redox chromophore being a viologen. The second substrate 114 is covered (apparently over the ITO layer thereon) with a nano-structured antimony-doped tin oxide film, and then with a white reflective layer made of titania. This titania layer is stated to be porous enough for lithium ions to pass therethrough, but the light-scattering properties of the titania produce a solid-white reflector. The display is then filled with the inert electrolyte 110.
FIG. 2 of the accompanying drawings is a flow chart of a method (generally designated 200) for preparing the display 100 shown in FIG. 1. In a first step 202, a colloidal titania dispersion is prepared by hydrolysis of titanium tetraisopropoxide; titanium tetrachloride may alternatively be used. The average diameter (7 nm) of the initially formed crystallites is increased to 12 nm by autoclaving at 200° C. for 12 hours. The autoclaved dispersion is concentrated to a solids content of 160 g/l and Carbowax 20000 (40% wt. equiv. of TiO2) is added to yield a white viscous sol. (Carbowax 20000, a Registered Trade Mark, is an ethylene glycol polymer of average molecular weight 20000.) The resultant sol is then, in a step 204, printed on to a glass substrate carrying a conducting layer. More precisely, a 4 μm thick layer of the sol, 25 mm by 25 mm in size, is deposited using a screen-printing printing technique on top of a 33 mm by 33 mm fluorine-doped tin oxide layer on a glass substrate formed of Glastron (Registered Trade Mark). The resulting gel-film is dried in air for 1 hour, sintered at 450° C. for 12 hours and stored in a darkened vacuum desiccator prior to use.
Separately, in a step 206, a redox chromophore, N,N′-bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride, is prepared by adding 4,4′-bipyridyl (4.4 g) and diethyl 2-ethylbromophosphonate (15.0 g) to water (75 ml). The reaction mixture is refluxed for 72 hours and allowed to cool. Following addition of concentrated hydrochloric acid (75 ml), the reaction mixture is refluxed for a further 24 hours. To recover the desired product, the reaction mixture is concentrated to 50 ml, isopropyl alcohol (200 ml) is added dropwise, and the mixture is stirred on ice for one hour and filtered. The white crystalline product is washed with cold isopropyl alcohol and air-dried to give pure N,N′-bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride.
In the next step 208 of the process, the redox chromophore prepared in step 206 is adsorbed on to the titania-coated substrate prepared in step 204 above. More precisely, the titania films are modified by adsorption of the redox chromophore from an aqueous solution (0.02 mol.dm−3) over 24 hours, washed with distilled de-ionized water, dried in air, and stored in a darkened vacuum desiccator for 48 hours prior to use.
The remaining steps of the process may be most easily understood by referring both to FIG. 2 and to FIGS. 3A and 3B. FIG. 3A shows, in schematic side elevation, an assembly (generally designated 300) being used to form the complete display 100 (except for the electrolyte 110). The assembly 300 comprises a front electrode assembly (FIG. 3A is inverted for ease of comprehension) consisting of a first glass substrate 302 (which will eventually form the glass substrate 102), a first tin-oxide coated conductive layer 304 (which will eventually form the conductive layer 104), and a nano-structured film of titania 306 (which will eventually form the film 106). The assembly 300 further comprises a rear electrode assembly, the construction of which will now be described with reference to FIGS. 3A and 3B.
FIG. 3B shows a top plan view of the pattern in which adhesive is applied to the second substrate prior to the final assembly. The first step 210 (FIG. 2) of the final assembly is the deposition, using a screen printing technique, of a 2.5 mm border 116 of a commercial epoxy resin (Araldite—Registered Trade Mark) on to a second 33×33 mm glass sheet 308 provided with a layer 310 of fluorine-doped tin oxide; this second glass sheet 308 eventually forms the second glass substrate 114, while the tin oxide layer 310 eventually forms the second conductive layer 112. As best seen in FIG. 3B, a small opening 118 is left in one corner of the border 116. The adhesive-coated piece of conducting glass is placed on top of the first glass sheet bearing the modified titania film prepared as described above and the resultant assembly is left for 24 hours to enable the adhesive to set, thus forming the final assembly 100, except for the absence of the electrolyte 110.
To complete construction of the electrochromic system, the above sandwich structure is, in a step 212, back-filled under argon pressure with an electrolyte solution consisting of lithium perchlorate (0.05 mol. dm−3) and ferrocene (0.05 mol. dm−3) in γ-butyrolactone (m. pt. −45° C., b. pt. 204° C.). The components of the electrolyte solution are carefully purified and rigorously dried prior to use. Finally, in a step 214, the opening 118 is closed using the same commercial epoxy adhesive as before.
In operation of the display 100, the nano-structured titania film 106 colors on application of a potential sufficiently negative to accumulate electrons in the available trap and conduction band states. According to the aforementioned U.S. Pat. No. 6,301,038, as a consequence of the high surface roughness of this nano-structured film, ions are readily adsorbed/intercalated at the oxide surface, permitting efficient charge compensation and rapid switching, i.e., the need for bulk intercalation is eliminated. However, despite the rapid switching times in such films, the associated change in transmittance is not sufficient for a commercial device. To overcome this limitation, redox chromophore 108 is adsorbed at the surface of the transparent nano-structured film; this chromophore, when reduced, increases the extinction coefficient of an accumulated trapped or conduction band electron by more than an order of magnitude. Furthermore, due to the nano-porous structure and associated surface roughness of the nano-crystalline films used, the redox chromophore is effectively stacked as in a polymer film, while at the same time the intimate contact of the chromophore with the metal oxide substrate necessary to ensure rapid switching times is maintained.
One potentially important market for electro-optic “displays” (or rather electro-optic systems) is windows with variable light transmission. As the energy performance of buildings and vehicles becomes increasingly important, electro-optic media could be used as coatings on windows to enable the proportion of incident radiation transmitted through the windows to be electronically controlled by varying the optical state of the electro-optic media. Effective implementation of such “variable-transmissivity” (“VT”) technology in buildings is expected to provide (1) reduction of unwanted heating effects during hot weather, thus reducing the amount of energy needed for cooling, the size of air conditioning plants, and peak electricity demand; (2) increased use of natural daylight, thus reducing energy used for lighting and peak electricity demand; and (3) increased occupant comfort by increasing both thermal and visual comfort. Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits but also (1) increased motoring safety, (2) reduced glare, (3) enhanced mirror performance (by using an electro-optic coating on the mirror), and (4) increased ability to use heads-up displays. Other potential applications include of VT technology include privacy glass, angle-independent high-contrast large-area displays, glare-guards in electronic devices, and electronic scratchpads.
In order to increase the usability of electrochromic displays, there are significant challenges to overcome that will allow for improved product life, lower costs, and wider manufacturing process conditions to produce a larger portfolio of product types without significant increase in process design and hence increased costs and delay of entry into the market.
A first aspect of the present invention seeks to reduce or eliminate one major limitation of the prior art electrochromic display described in the aforementioned U.S. Pat. No. 6,301,038, WO 01/27690 and Wood article, namely that great care must be taken to contain the liquid electrolyte and to create the seal. This step is expensive and can lead to product defects if the seal leaks, which is possible with changes in temperature (temperature cycling).
This first aspect of the present invention also seeks to reduce or eliminate another limitation of such prior art electrochromic displays, namely that the switching response of the display is limited to the speed at which the ions can transport across the electrolyte, and that therefore, such displays are limited to slow speed applications.
This first aspect of the present invention also seeks to reduce or eliminate another limitation of liquid electrolyte electrochromic displays, namely that it is difficult to create isolated cells unless an electrolyte seal is used for each display cell element, which both causes great expense and limits density (i.e., how closely display cells can be packed). When the cells are close together, ion transport of one cell can create ion transport of neighboring cells and therefore can cause the neighboring non-activated cells to become somewhat activated, resulting in a loss of clarity or resolution. In order to overcome this in the current art, the cells are not packed closely together, preventing optimized resolution.
The first aspect of the present invention seeks to provide to create an electrochromic display that is low cost, high density, fast, and long lasting, but which does not need a liquid electrolyte.
The first aspect of the present invention also seeks to provide a structure and method of making the structure that has a high-speed solid charge transport layer between the display conductive elements and the redox promoter or chromophore. (Note that in such a display holes must move when electrons are used to produce the electrochromic effects. If oxidation is used to produce the electrochromic effects, the charge transport layer can be an electron transport layer. Of course, holes moving in one direction are equivalent to electrons moving in the other.)
The first aspect of the present invention also seeks to provide a structure and method of making the structure that has a high-speed solid charge transport layer between the display conductive element and the redox promoter or chromophore where the redox promoter or chromophore is improved for efficiency by chemically adding a charge transport polymer to the redox promoter or chromophore prior to adding the solid charge transport layer.
The first aspect of the present invention also seeks to provide a structure and method of making the structure that has a high-speed solid charge transport layer between the display elements and the redox promoter or chromophore where the solid charge transport layer is patterned and aligned to the display elements.
A second aspect of the present invention is directed to increasing the usability of electrochromic displays in VT and other applications by making the displays more readable by providing better contrast between the on/off states, as well as better contrast between different gray levels. Currently electrochromic displays are limited in contrast, because although the redox chromophores change color when ion transport occurs, they do so against a clear electrolyte, and a thin (3-4 μm) roughened surface, which is not optimized for contrast, since it is chosen to enhance the amount of surface area for the connection of redox chromophore and not for particular optical properties.
This second aspect of the present invention also seeks to increase the range of applications of electrochromic displays by providing a low-cost electrochromic display manufacturing process that integrates with other processes such as flexible polymer substrate processes is required. Currently, electrochromic display manufacturing processes are time consuming (>12 hours of preparation time) and therefore are costly. In addition, because of the high-temperature (450° C.) titania sintering step, these processes do not integrate well with low-temperature plastic-based processes for producing flexible displays.
The second aspect of the present invention seeks to provide a structure for and method of making low-cost electrochromic displays. This second aspect also seeks to provide a structure for and method of making electrochromic displays that has at least one of the following advantages: (a) less time consuming; (b) more easily integrated with other manufacturing processes; (c) providing displays that are more readable and have enhanced contrast; (d) providing displays that have a very high surface roughness of nano-structured film to further enhance contrast; and (e) providing displays that have both an improved electrolyte and a very high surface roughness of nano-structured film to further enhance contrast.
A third aspect of the present invention relates to reducing the susceptibility of electrochromic displays to environmental factors. Application of electrochromic media in VT windows will necessarily expose the media to substantial variations in environmental conditions, and in the present state of electrochromic technology, the applications of electrochromic media are, the present inventors have realized, significantly limited by the susceptibility of such media to many environmental factors, such as light, moisture, oxygen, and electrostatic discharge. Reducing the susceptibility of the media to such factors would allow for improved product life, lower costs, and wider manufacturing process conditions to produce a larger portfolio of product types without significant increase in process design and hence increased costs and delay of market entry.
One major limitation of electrochromic media, the present inventors have realized, is the susceptibility of the redox chromophore to light degradation. Under excessive ultra-violet (UV) radiation exposure (in terms of total flux by either high doses or lower doses over long periods), the redox chromophore may degrade or detach from the nano-structured titania film. Titania is notorious for its photocatalytic ability, under UV illumination, to catalyze the oxidation of organic materials, typically to carbon dioxide and water. Indeed, titania is used for precisely this reason in anti-microbial materials in air conditioning filters and medical devices, waste water treatment and air decontamination. Such photocatalytic oxidation of organic materials is of course enhanced as the concentration of molecular oxygen in the medium increases. The photocatalytic oxidation is also enhanced as the concentration of water in the medium increases, since titania can photocatalytically split water to produce hydrogen and oxygen, so that the presence of water inherently produces an increase in oxygen concentration. Furthermore, it is known that water may act as a nucleophile under certain conditions and, in the presence of titania, water may nucleophilically attack the aromatic groups present in the chromophore, producing products of unknown chemistry which are likely to have properties significantly different from those desired in the chromophore.
The susceptibility of components of the electrochromic medium to degradation by oxygen in the presence of titania is enhanced by the oxidation and reduction reactions which take place at the electrodes during switching of the state of the medium, since it is well known that many compounds are more susceptible to such degradation reactions as they are being generated at an electrode.
Another problem with electrochromic media, the present inventors have realized, is their susceptibility to damage by discharges of static electricity, such as triboelectric charges built up during assembly of a display or its handling as it is moved to a desired location. Static electricity is typically of very high voltage, orders of magnitude larger than the relatively small voltages, around 1 to 2 Volts, needed to switch electrochromic media. Static discharges can damage electronic parts and affect components of the media, especially where the discharge passes through interfaces. Although the exact reactions involved are not completely understood, exposure of electrochromic media to large “over potentials” beyond the working voltage of the media can degrade the redox chromophore by oxidation or reduction. Under such high voltage discharges, the aromatic chromophores may be altered by, for example, intramolecular cyclizations and rearrangements and/or intermolecular coupling or other similar reactions with adjacent chromophores.
Another possible problem with electrochromic media, the present inventors have realized, is interaction between pixels. The aforementioned U.S. Pat. No. 6,301,038 describes direct drive electrochromic displays having multiple pixels integrated together using a common electrolyte seal. The seal used in the prior art to keep the electrolyte inside the display is the border of epoxy resin. Because groups of pixels use a common electrolyte and because fields of one pixel are not well isolated from neighboring pixels, these non-isolated fields could create ion transport near neighboring pixels. Hence, these non-isolated fields may modify the redox chromophore and further degrade the lifetime of the redox chromophore in that region (e.g., redox chromophore designed to be “off” is “on” to a certain level systematically more often than it was designed to be).
Another problem with electrochromic media, the present inventors have realized, is high fields caused by a lack of height or distance control within an electrochromic display between the glass substrates, for example as shown in FIG. 1. A user who presses on the first glass substrate moves the display surfaces closer together and, when in operation, the field strength then increases significantly because the field is proportional to the inverse of the distance. Hence, the electrodes and other components of the display can degrade due to arcing, shorting, or excessive ion transport.
The third aspect of the present invention seeks to provide a means for and method of making a total environmental seal for electrochromic displays for sealing out unwanted light, unwanted moisture, unwanted oxygen (O2), unwanted electrostatic charge, and unwanted fields.
The first, second and third aspects of the present invention discussed above all relate to improvements in electrochromic displays. However, the present invention has a fourth aspect which is applicable to all electro-optic displays, not merely electrochromic displays, and this fourth aspect of the present invention relates to a system and method of operation for integrating and controlling an electro-optic display.
As already mentioned electrochromic devices have been in use for some time in relatively simple applications such as the electrochromic rear view mirrors for motor vehicles. These electrochromic mirrors change from the full reflectance mode (day) to the partial reflectance mode(s) (night) for glare-protection purposes from light emanating from the headlights of vehicles approaching from the rear. However, recent research into the production of nano-crystalline electrochromic display elements based on chemically modified nano-structured meso-porous films indicates that electrochromism can be extended to the high density electronic display market and can favorably compete with LCD's in certain applications.
Also as already mentioned, U.S. Pat. No. 6,301,038 describes an electrochromic display in which the active layer (i.e., the layer whose optical characteristics are varied by addition or removal of electrons is a nano-porous-nano-crystalline film comprising a semi-metallic oxide having a redox chromophore adsorbed thereto. Also as already mentioned, this patent describes a nano-crystalline electrochromic system comprising a first electrode disposed on a transparent or translucent substrate and a second electrode, an electrolyte, an electron donor and a nano-porous-nano-crystalline film of a semiconducting metallic oxide having a redox chromophore adsorbed thereto being intermediate the first and second electrodes. In FIG. 4 is illustrated a nano-crystalline electrochromic display cell 400 similar to those described in this patent. Nano-crystalline electrochromic display cell 400 consists of a first glass substrate 410, a fluorine-doped tin-oxide coated conductive layer 420, a terminal 425, a nano-structured film of titania 430, a redox chromophore 440, an electrolyte solution 450, a conductive element 460, a terminal 465, a conductive element 470, a terminal 475, and a second glass substrate 480.
Redox chromophore 440 is colorless in the oxidized state and colored in the reduced state. It is linked to the surface of nano-structured titania 430, a nearly colorless semiconductor, on the first glass substrate 410. When a current is allowed to flow from terminal 465 to terminal 425, electrons are injected from first glass substrate 410 into the conduction band of the semiconductor nano-structured titania film 430, and this current reduces the redox chromophore 440. This reduction is reversible: application of a reverse current re-bleaches the redox chromophore 440 by oxidation.
A major potential advantage of nano-crystalline electrochromic display technology is the ability to create a true “paper-like” display. A combination of high reflectivity and achievable contrast ratio gives the nano-crystalline electrochromic display an appearance that is more like ink on paper than most other display technologies available and that can be read at very large angles to the perpendicular (again, like paper). An additional advantage is that nano-crystalline electrochromic displays can be made bi-stable, meaning that once switched on, a pixel will stay colored until actively bleached. In other words, no power is consumed to keep a pixel colored. This, combined with the fact that the display is reflective, needing no backlighting, means that the displays can be designed to require very little power to operate. This could provide a significant market advantage in handheld, battery operated electronic devices such as cellular phones, PDA's, and electronic books.
Nano-crystalline electrochromic-based display cells have been studied as individual cells, under simple direct drive operation, whereas none of the known art discusses active matrix displays. Indeed, it is not quite clear how to make active matrix nano-crystalline electrochromic displays since a common electrolyte that is used between cells may cause interference between cells in an active matrix.
Even in direct-drive nano-crystalline electrochromic displays, the system (electronics/software/display) and method of operation are complex because, as in the electrophoretic display cell discussed earlier, the state of the cell before it is written is critical in determining how to change the cell state.
FIG. 5 shows a simplified electrical circuit model (generally designated 500) of the nano-crystalline electrochromic display cell 400 shown in FIG. 4. Electrical circuit model 500 consists of terminal 465 and terminal 425 as described above with reference to FIG. 4, a resistor 510, a resistor 520, and a capacitor 530.
Resistor 510 represents the summed series impedance of terminal 465, terminal 425, conductive layer 420, nano-structured titania film 430, redox chromophore 440, conductive element 460, and terminal 465. Capacitor 530 represents the capacitance of electrolyte solution 450. Resistor 520 represents the sum of the impedance as represented by resistor 510 and the series impedance of electrolyte solution 450. Resistor 520 has a characteristic impedance that is much larger than that of resistor 510. It should be noted that a considerably more complex small-signal model could be developed for nano-crystalline electrochromic display cell 400. Of special interest would be inclusion of the variable diodic behavior of nano-structured titania film 430.
In order to take competitive advantage of the potential of electro-optic display technologies, and in particular of the encapsulated electrophoretic, rotating bichromal member and electrochromic display technologies described above, the displays need to be more readable, and hence require better contrast between the “on/off” states as well as better contrast between different “gray levels”. A key technique in achieving this is to drive the electro-optic display in a manner which takes into account environmental factors. For example, the nano-crystalline electrochromic display cell 400 should be driven in a manner that takes into account many factors. The nano-crystalline electrochromic display cell 400 drive system must account for (1) the steady-state response of resistor 510, resistor 520, and capacitor 530; (2) the time-varying response of resistor 510, resistor 520, and capacitor 530; (3) interactions with adjacent cells; (4) light reflectivity vs. ion transport curve for redox chromophore 440; (5) ion transport efficiency; (6) variable diodic behavior of nano-structured titania film 430; (7) interaction of optical feedback; (8) electrolyte potential changeover; (9) changes over operating life of electrolyte solution 450 performance; (10) changes over operating life of redox chromophore 440 performance; (11) effects of ambient and operating temperatures on cell performance; and (12) effects of ambient light on display quality.
Similarly, it has been observed that the optical characteristics of encapsulated electrophoretic media vary as a function of temperature and humidity, and the “age” of the medium; this aging phenomenon is affected by both the chronological age of the medium, that it to say the period since the medium was manufactured, and the “operating age”, that is to say the period for which the medium has been driven. More specifically, the electrical resistivity of an encapsulated electrophoretic medium varies inversely with temperature, decreasing as the temperature increases. This variation of electrical resistivity with temperature affects how much current passes through the medium when it is driven with a constant drive pulse, and this is turn affects the rate at which the medium switches and the rate at which the medium ages during use. Thus, using a fixed drive pulse with an encapsulated electrophoretic medium which is undergoing changes in ambient temperature and humidity, and is also aging, can lead to substantial and undesirable changes in the optical properties of the medium. Such changes can include the reflectances of the white and dark states of the medium and the intermediate gray states (if any), and hence also the contrast ratio of the medium. For example, the medium may show acceptable properties when switched at room temperature, but its contrast ratio may be reduced when operating at low temperatures, and the medium may be over-saturated and/or over-driven at high temperatures; such over-driving is undesirable because it tends to reduce the working lifetime of the medium.
Changes in environmental conditions may also cause problems with self-erasing of the medium. “Self-erasing” (see, for example, Ota, I., et al., “Developments in Electrophoretic Displays”, Proceedings of the SID, 78, 243 (1977), where self-erasing was reported in an unencapsulated electrophoretic display) is a phenomenon whereby, when the voltage applied across certain electrophoretic media is switched off, the electrophoretic medium may reverse its optical state, and in some cases a reverse voltage, which may be larger than the operating voltage, can be observed to occur across the electrodes adjacent the medium. It appears (although this invention is in no way limited by this belief) that the self-erasing phenomenon is due to a mismatch in electrical properties between various components of the display. Obviously, self-erasing is highly undesirable in that it reverses (or otherwise distorts, in the case of a grayscale medium) the desired optical state of the medium.
Similar problems are encountered with other types of electro-optic media. For example, the switching characteristics of rotating bichromal member media will vary with temperature due to changes with temperature in the viscosity of the liquid medium which surrounds the rotating bichromal members, and such temperature-dependent changes may affect the gray scale of the medium.
In its fourth aspect, the present invention seeks to provide a system and method of operation for integrating and controlling an electro-optic display.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display to provide controlled dark and white states, and controlled intermediate states (gray scale) in displays capable of such gray scale.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display for optimum steady-state performance.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display for optimum time-varying performance.
The fourth aspect of the present invention also seeks to electrically drive a nano-crystalline electrochromic display cell that compensates for interference from adjacent cells.
The fourth aspect of the present invention also seeks to electrically drive a nano-crystalline electrochromic display cell that compensates for variations in light reflectivity vs. ion transport curve for redox chromophores.
The fourth aspect of the present invention also seeks to electrically drive a nano-crystalline electrochromic display cell that compensates for variations in ion transport efficiency.
The fourth aspect of the present invention also seeks to electrically drive a nano-crystalline electrochromic display cell that compensates for variations in diodic behavior of a nano-structured titania film.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display that compensates for the interaction of optical feedback.
The fourth aspect of the present invention also seeks to electrically drive a nano-crystalline electrochromic display cell that compensates for electrolyte potential changeover.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display in a manner that incorporates the material aging aspects of the electro-optic medium.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display in a manner that compensates for the effects of ambient and operating temperatures on electro-optic medium performance.
The fourth aspect of the present invention also seeks to electrically drive an electro-optic display in a manner that compensates for ambient light conditions.